[vages]

You could have asked the same question in 1700: What will the state do with its inhabitants when they no longer work the fields or make wool? The answer is “something else”. The economy adapts, and unimaginable professions and social structures appear. I would be amazed if anyone in 1700 thought that programmer could be a profession.

And maybe we don't need as many as 6 billion people on earth, maybe one billion will do (the slow population growth in industrialized countries indicates that depopulation is possible). But the need for them won't disappear overnight; we are nowhere near artificial general intelligence, no matter what Ray Kurzweil tells you.

[bilbo0s]

>the slow population growth in industrialized countries indicates that depopulation is possible

Slow population growth, does not lead to depopulation.

(Not that we would need depopulation anyway. More just a point that slow growth, is still growth.

[Engineering-MD]

Your point is, of course correct: slow growth is growth. However, it worth clarifying that population growth in much of Europe is driven by immigration rather than natural growth, and therefore does demonstrate population decline which is offset by other regions population growth.